This panino is named for its bread and nothing else, which tells you the bread is the decision. A sfilatino is a thin, long Italian loaf, slender and crisp-crusted, closer to a slim baton than to a full filone: a high ratio of crackling crust to a modest interior crumb. Choosing it as the carrier sets the rules for everything that goes inside. The thin profile means the filling is laid in a long, even line rather than stacked, and the crisp shell means the sandwich reads as much through the snap of the bread as through whatever it holds. It is the everyday Italian roll in its leanest form, the carrier you reach for when you want crust to be part of the bite.
The craft is matching the filling to a bread that is mostly crust. Because the crumb is thin, a sfilatino cannot absorb a wet, oily filling without going limp at the seam, so it favours drier components laid in a clean line: a few slices of salume, a firm cheese, a leaf or two, dressed lightly if at all. It wants filling close to eating, since the thin crust that is its whole appeal also stales fast once cut and dressed. Split lengthwise and sometimes passed under heat to re-crisp the cut face, it carries the filling along its length so every bite has the same ratio. The discipline is restraint in volume: overfill a thin loaf and the crust shatters in the wrong direction. It is eaten soon, while the shell still cracks.
The variations are really the rest of the Italian bread map, since the sfilatino is one carrier among many. The same fillings move onto a hollow rosetta, a chewy ciabatta, a dense pane casereccio, and each loaf changes the sandwich more than the filling does. Those breads, and the place-named panini built on them, each set their own rules and deserve their own articles rather than being crowded in here.